measuring heater voltage

[chopchip]

im building a tube preamp from a decware kit, when i measure the voltage between pin 4 or 5 i get 3.5 volts, the tubes are 6dj8

is this correct?

thanks

 

[kmtang]

PIN 4 AND 5 is the filament with 6.3V ac or dc. Check the AC voltage input to the bridge rectifier and whether there are anything wrong with the rectifier itself.


Johnny

 

[bob91343]

A 6DJ8 will work okay down to 5 or 5.5 V but yours is too low. If it's on ac maybe you are running to the center tap; if it's dc then Johnny's suggestion should be followed.

 

[chopchip]

it is ac and i dont have the rectifier installed because i dont want to charge the capacitors.

there are three wires two green and one green/yellow. the green yellow is the center tap and it goes to ground the other wires run to the heater pins 4 and 5.

is this correct?

i also didnt bother routing the legs from pin 4 to pin 4 etc on all tubes. does this make a difference?

thanks

 

[bob91343]

Is it possible you are using a 240V transformer on a 120V mains circuit? The voltage is too low. No, it doesn't matter if pin 4 is routed to the other pins 4.

 

[chopchip]

quoting steve decker the designer of this transformer

"For 120 Volt operation of the world voltage transformer you connect the black wire and white wire together - That's your LINE. Then connect the black wire with white stripe to the brown wire - That's you NEUTRAL.

The brown wire with the white stripe and the pink wire will be unused and should be capped off.

We decided recently to upgrade the ZSP1 preamp kit with the world voltage transformer and I forgot to update the schematic, sorry.

Steve"


so yes it is a 240v as well as a 120

 

[kevinkr]

Measure the voltage between pins 4 & 5 of your 6DJ8 and not ground - not clear to me if you measured to ground or pin to pin..

If indeed you did measure pin to pin it probably is an indication that you have wired the primaries of your transformer for 240V... Be careful if this is the case and make sure you have the primaries phased correctly before plugging in. Use a 25 - 60W lamp in series with the mains feeding the transformer which will prevent you from blowing the fuse (you do have a fuse don't you??) and protect the transformer from melt down if you don't. (Lamp at full brightness is a very good clue that something is wrong - probably primaries not phased correctly.)

Be very careful.. I sense a newbie here.. 😀

 

[chopchip]

i allready blew a fuse. i measured from pin to ground.

im a novice. but im still breathing.

thanks

 

[chopchip]

so i measured from pin to pin and i get more than 7 volts.
ok
thanks
sorry
silly me hehehehehe.

so if i want to bring the voltage down to 6 volts and each tube draws 300ma then 1.2 amps for the lot. they are wired in parallel. so R=1/1.2 R=.83 ohms

so i would use a small value resistor like that? is that correct?

thanks

 

[kevinkr]

so i measured from pin to pin and i get more than 7 volts.
ok
thanks
sorry
silly me hehehehehe.

so if i want to bring the voltage down to 6 volts and each tube draws 300ma then 1.2 amps for the lot. they are wired in parallel. so R=1/1.2 R=.83 ohms

so i would use a small value resistor like that? is that correct?

thanks

Yes that would be correct, use the closest value 3 - 5W resistor, however a quick question..

Have you applied plate voltage yet? If the transformer is not fully loaded the filament voltage might be a bit high.. If in fact you have plate voltage applied to all of the stages then it would be a good idea to go ahead and use a series resistor to lower the voltage. 7V is actually rather high..

 

[Soonerorlater]

Hi chopchip, similar question to kevinkr only I was too slow 🙂


Before you think about using a dropping resistor you might need to measure the heater voltage with all the valves in place to provide a load for the heater transformer winding. Off load voltage might be 10% higher ie nearly 7v

Brgds Bill

 

[bob91343]

I don't think you have to drop the heater voltage; by the time you add the other heaters it will sag to a reasonable value. As long as it's well below 7V during operation you won't have any trouble. And realize the correct voltage is not 6V, it's 6.3V. The reason? Because many tubes were expected to be used in car radios when everyone's car had 6V batteries.

 

[chopchip]

Yes that would be correct, use the closest value 3 - 5W resistor, however a quick question..

Have you applied plate voltage yet? If the transformer is not fully loaded the filament voltage might be a bit high.. If in fact you have plate voltage applied to all of the stages then it would be a good idea to go ahead and use a series resistor to lower the voltage. 7V is actually rather high..

ok thanks. no i havent completed the build yet. so i will test it again later.

thanks for the advice!